Polyimide resins have high heat resistance and excellent electrical characteristics, and one major application thereof is a protective film for semiconductor devices and an interlayer dielectric film in multilayer interconnection structures.
Conventionally, the protective film for semiconductor devices and the interlayer dielectric film in multilayer interconnection structures have been formed by using photoresist. Thus, a polyamide acid, a precursor of a polyimide resin, is applied on a substrate, and heated to form a polyimide film. Then, on the surface of the polyimide film, a photoresist film is mounted, and this photoresist film is exposed to light and developed to form a resist pattern. Then using this resist pattern as an anti-etching mask, the selective etching of the base polyimide film is carried out. In this way, a polyimide protective film or an interlayer dielectric film having the desired pattern can be formed.
However, when a resin pattern is formed using the above method, two steps of forming a polyimide film and of forming a resin pattern must be carried out. In order to solve such a problem, polyimides or polyimide precursors having a photosensitive characteristics are being developed.
For example, it has been investigated to obtain a photosensitive composition by blending a polyamide acid comprising an aromatic acid dianhydride and an aromatic diamine with a naphthoquinone diazide compound as a photosensitizer (Japanese Unexamined Patent Publication (Kokai) No. 52-13315). Said photosensitive composition is a positive type in which the UV-irradiated region is dissolved in an alkali developing solution. Thus, a naphthoquinone diazide compound forms an indene carboxylic acid by absorbing a UV ray, thereby enhancing the dissolution rate into the alkali developing solution in the exposed region. On the other hand, non-exposed region not irradiated with a UV ray becomes difficult to dissolve in an alkali developing solution due to the hydrophobicity of the naphthoquinone diazide compound. As a result, due to the difference in solubility of the exposed region and the nonexposed region, a line image can be formed. However, though the nonexposed region cannot be easily dissolved in an alkali developing solution due to the hydrophobicity of naphthoquinone diazide, it dissolves out due to the high solubility of the polyamide acid, leading to film reduction, and thus a sufficient line image cannot be obtained.
As a method of suppressing film reduction, a method (Japanese Unexamined Patent Publication (Kokai) No. 62-135824) has been devised in which a part of the carboxyl groups of a polyamide acid is neutralized with a strong base triethylamine, and the coated film is heated to imidize 10-20% of the polyamide acid thereby to suppress the dissolution of the nonexposed region. However, in this method, the control of imidization with an amine compound is difficult, and a line image cannot be obtained in a reproducible manner. Furthermore, it has a problem of low storage stability of varnish. Also, this requires the use of an aqueous potassium hydroxide solution after developing, and thus developing with an aqueous sodium carbonate solution which is a recent demand is difficult.
Furthermore, it has been investigated to blend a photosensitive composition comprising a polyimide precursor, a naphthoquinone diazide compound, with a basic nitrogen-containing compound of which base dissociation constant (pKb) at 25° C. is 7 or greater (Japanese Unexamined Patent Publication (Kokai) No. 2003-5369). This method intends to suppress the solubility of the UV-nonirradiated portion by neutralizing a part of the carboxyl group borne by the polyimide precursor. However, even with this method, when an amine basic compound is to be blended, there is a problem of deteriorating the storage stability of varnish. Also, since imidation gradually proceeds in the heating step such as the desolvation step and the lamination step, a line image cannot be obtained in a reproducible manner.
Efforts are being made to reduce the solubility into the alkali developing solution by esterifying the carboxyl group of a polyamide acid with a photopolymerizable group. For example, a photopolymerizable group has been introduced by an ester bond with the carboxyl group in the synthetic process of a polyamide acid to obtain a negative type photosensitive resin composition (Japanese Unexamined Patent Publication (Kokai) No. 49-115541). However, the synthesis of the above photosensitive resin composition involves a number of steps, and is troublesome. Also, since an organic solvent is used in the developing solution, the exposed portion may also swell by the organic solvent, and thus a sufficient line image may not be obtained.
In recent years, a film-type print-circuit board called a flexible printed circuit board (hereinafter referred to as FPC) is drawing much attention. FPC is has mainly been used in mobile phones, laptop personal computers, digital cameras and the like. Since FPC remains active even when it is bent, it is an indispensable material for miniaturized and light-weighted devices. Specifically in recent years, miniaturized and light-weighted devices are gaining popularity, and accordingly FPC is contributing to reduced dimension and reduced weight of the above devices, reduced production cost, simplified designing etc.
As a coverlay material for FPC, an adhesive-attached polyimide film has been used in terms of bending resistance, thermal resistance and electric insulating properties. However, when the polyimide film is to be used, there are problems in the precision of position adjustment and cost since it is stuck and punched by the human hand.
Thus, various methods are being devised that form a polyimide in which after a photosensitive property is imparted to a polyimide precursor and a wiring pattern is formed, the polyimide precursor is heated to form the polyimide (for example, U.S. Pat. No. 3,064,579, Japanese Unexamined Patent Publication (Kokai) No. 2002-278061). However, when these polyimide precursors are to be imidated, temperature as high as 300° C. or more is required. Thus, for applications that do not tolerate high temperature, for example a coverlay material provided to FPC for use in electronic devices, the method cannot be applied.